Perth Geography and Weather

Geography

The geography of a locality plays a significant role in the design of airspace
and traffic handling procedures. The prevailing winds and their intensity
dictate runway direction and length, while the location of population centres,
other airports, vegetation, hills and mountains affect approach and departure
procedures.

The city of Perth is
located on a narrow coastal plain, set against an escarpment, the Darling
Range, which runs parallel to the coast.
Perth airport is east of the city, close to the escarpment. Significant turbulence is caused by the
passage of easterly winds over the escarpment.

RAAF Base Pearce is located in close proximity to the north of Perth,
which impacts on approaches and departures at both airfields, while Jandakot is
located under the flight path to the southwest.

Weather

Perth enjoys a Mediterranean style climate of mild wet winters and hot, dry
summers.Operations at Perth are affected
by the passage of fronts in winter, low pressure troughs in summer, easterly
winds, the sea breeze, thunderstorms and cyclones.

Sea Breeze - the “Fremantle Doctor”

The South-West area of WA is subject to significant sea breeze activity during
the summer months. The prevailing wind is a strong, gusty, hot easterly, but
during the day the land heats rapidly and a strong sea breeze crosses the
coast.

The sea breeze occurs over two
thirds of the days between the months of November to February, and can reach
wind speeds of more than 20knots. As the
easterly wind and the sea breeze battle for supremacy, there may occur several
closely spaced runway changes at Perth.
A significant low-level wind shear can develop as the cooler air cuts
under the warmer easterly.

Because it is closer to the coast,
Jandakot Tower can give 30 minutes or so notice to Perth Tower of the sea
breeze’s arrival. Passed onto Perth TMA
and Melbourne Centre, this is a significant benefit for traffic planning. The
easterly wind, as it passes over the Darling Range, causes significant
turbulence at Perth airport.

Thunderstorm Development

The major weather influences during summer in Perth are the sea breeze and the
development of a low-pressure trough down the west coast. A typical
summer situation is the formation of a large high-pressure cell in the Great
Australian Bight, which directs hot and gusty easterly winds from inland
Australia to Perth. As a trough
starts to develop down the west coast, temperatures rise into the 30'sC° and a
late weak sea breeze develops.

During the next 24 hours the trough
of low pressure down the west coast starts to deepen, the wind backs to the
northeast and the mornings are hot, with an earlier sea breeze bringing some
relief.

The position of the
trough is the deciding factor in how hot it gets and what time the sea breeze
reaches the city. It can mean the difference of a 40 C° day or a much cooler
day of 25 C° - 30C°.

However, the trough can become active and produce thunderstorms and extreme rainfall, particularly to the north and east of Perth.

A "tropical cyclone" is a tropical low-pressure system intense enough
to produce sustained gale force winds (at least 63 km/h). A "severe
tropical cyclone" produces sustained hurricane force winds (at least 118
km/h), and corresponds to the hurricanes or typhoons of other parts of the
world.

In the average cyclone season, 10
tropical cyclones develop over Australian waters, of which six cross the coast,
mostly over northwest Australia (between Exmouth and Broome), and northeast
Queensland (between about Mossman and Maryborough).

In April 1978, Cyclone Alby passed over Perth,
killing 5 people and causing severe storm damage and bushfires to the southwest
of WA.